Military robots come in an astonishing range of shapes and sizes. DelFly, a dragonfly-shaped surveillance drone built at the Delft University of Technology in the Netherlands, weighs less than a gold wedding ring, camera included. At the other end of the scale is America’s biggest and fastest drone, the $15m Avenger, the first of which recently began testing in Afghanistan. It uses a jet engine to carry up to 2.7 tonnes of bombs, sensors and other types of payload at more than 740kph (460mph).

On the ground, robots range from truck-sized to tiny. TerraMax, a robotics kit made by Oshkosh Defense, based in Wisconsin, turns military lorries or armoured vehicles into remotely controlled or autonomous machines. And smaller robotic beasties are hopping, crawling and running into action, as three models built by Boston Dynamics, a spin-out from the Massachusetts Institute of Technology (MIT), illustrate.

By jabbing the ground with a gas-powered piston, the Sand Flea can leap through a window, or onto a roof nine metres up. Gyro-stabilisers provide smooth in-air filming and landings. The 5kg robot then rolls along on wheels until another hop is needed—to jump up some stairs, perhaps, or to a rooftop across the street. Another robot, RiSE, resembles a giant cockroach and uses six legs, tipped with short, Velcro-like spikes, to climb coarse walls. Biggest of all is the LS3 (pictured above), a four-legged dog-like robot that uses computer vision to trot behind a human over rough terrain carrying more than 180kg of supplies. The firm says it could be deployed within three years.

Demand for land robots, also known as unmanned ground vehicles (UGVs), began to pick up a decade ago after American-led forces knocked the Taliban from power in Afghanistan. Soldiers hunting Osama bin Laden and his al-Qaeda fighters in the Hindu Kush were keen to send robot scouts into caves first. Remote-controlled ground robots then proved enormously helpful in the discovery and removal of makeshift roadside bombs in Afghanistan, Iraq, and elsewhere. Visiongain, a research firm, reckons a total of $689m will be spent on ground robots this year. The ten biggest buyers in descending order are America, followed by Israel, a distant second, and Britain, Germany, China, South Korea, Singapore, Australia, France and Canada.

Robots’ capabilities have steadily improved. Upload a mugshot into an SUGV, a briefcase-sized robot than runs on caterpillar tracks, and it can identify a man walking in a crowd and follow him. Its maker, iRobot, another MIT spin-out, is best known for its robot vacuum cleaners. Its latest military robot, FirstLook, is a smaller device that also runs on tracks. Equipped with four cameras, it is designed to be thrown through windows or over walls.

Another throwable reconnaissance robot, the Scout XT Throwbot made by Recon Robotics, based in Edina, Minnesota, was one of the stars of the Ground Robotics Capabilities conference held in San Diego in March. Shaped like a two-headed hammer with wheels on each head, the Scout XT has the heft of a grenade and can be thrown through glass windows. Wheel spikes provide traction on steep or rocky surfaces. In February the US Army ordered 1,100 Scout XTs for $13.9m. Another version, being developed with the US Navy, can be taken to a ship inside a small aquatic robot, and will use magnetic wheels to climb up the hull and onto the deck, says Alan Bignall, Recon’s boss.

Even more exotic designs are in development. DARPA, the research arm of America’s Department of Defence, is funding the development of small, soft robots that move like jerky slithering blobs. EATR, another DARPA project, is a foraging robot that gathers leaves and wood for fuel and then burns it to generate electricity. Researchers at Italy’s Sant’Anna School of Advanced Studies, in Pisa, have designed a snakelike aquatic robot. And a small helicopter drone called the Pelican, designed by German and American companies, could remain aloft for weeks, powered by energy from a ground-based laser.